Vehicle brakes

ABSTRACT

An auto-adjuster mechanism for an internal shoe drum brake of the type having an adjustable length strut through which handbrake actuating forces are transmitted from a handbrake mechanism to the shoes is characterized by a pawl and ratchet adjuster of which the pawl is pivotally mounted and is subjected to the action of a coil spring stressed both in torsion (to urge the pawl in a direction to effect adjustment) and in compression (to urge the pawl yieldingly into engagement with the ratchet) and only operates to effect adjustment of the strut during operation of the service brake means.

United States Patent 1 Margetts 1 March 6, 1973 [54] VEHICLE BRAKES [75]Hugh Grenville mingham, England Girling Limited, Birmingham, EnglandFiled: Aug. 23, 1971 Appl. No.: 173,989

Inventor: Margetts, Bir- [73] Assignee:

[30] Foreign Application Priority Data Aug. 24, 1970 Great Britain..4o,s73/7o [56] References Cited 3,621,947 Margetts 1 88/795 P PrimaryExaminer-Duane A. Reger Attorney--Scrivener, Parker, Scrivener andClarke 571 ABSTRACT An auto-adjuster mechanism for an internal shoe drumbrake of the type having an adjustable length strut through whichhandbrake actuating forces are transmitted from a handbrake mechanism tothe shoes is characterized by a pawl and ratchet adjuster of which thepawl is pivotally mounted and is subjected to the action of a coilspring stressed both in torsion (to urge the pawl in a direction toeffect adjustment) and in compression (to urge the pawl yieldingly intoengagement with the ratchet) and only operates to effect adjustment ofthe strut during operation of the service brake means.

3 Claims, 5 Drawing Figures 10 Q rs SHEET 10F 4 FIG].

PATENTED R 6 75 I SHEET 30F 4 SHEET Q [If 4 PATENTEDHAR 5 m VEHICLEBRAKES The Complete Specification of our prior British Patent Number1,199,191 described and claims an internal shoe drum brake including apair of brake shoes which can be expanded, against the action of shoereturn spring means, alternatively by power actuated service brake meansor by manually actuated auxiliary brake means, a strut of adjustablelength which transmits manually applied loads between the shoes but isrelieved of load upon actuation of the service brake means, the strutcomprising a pair of relatively rotatable members screw-threadedtogether, and pawl and ratchet means for effecting relative rotation ofthe members to lengthen the strut in response to excess travel of thebrake shoes upon brake actuation, wherein such relative rotation iseffected only when the service brake is operated to relieve the loadimposed on the strut by the return spring means.

The present invention relates to additions or modifications to brakes ofthis form, and provides such a brake in which a pawl spring in the formof a coil spring pre-stressed in torsion and in compression acts on thepawl on the one hand to bias it to rotate in a sense to increase thelength of the strut and on the other hand urge the pawl laterally intoengagement with the ratchet, the rotational force applied to the pawl bythe spring being intermediate the force required to overcome thefrictional resistance to rotationof the strut parts when the strut issubjected to load by the shoe return springs and the force required toeffect such rotation when the shoe return spring load is relieved byservice brake actuation.

A preferred embodiment of the present invention is described below, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a side elevation of one form of drum brake in accordance withthe invention;

FIGS. 2 and 3 are sections in the lines IIII and III- Ill of FIG. 1;

FIGS. 4 and 5 are plan and side elevations on an enlarged scale of partof the brake The brake illustrated in the accompanying drawings isgenerally similar in construction and operation to that described in theComplete Specification of British Patent No. 1,199,191, with particularreference to FIGS. 3, 4, and 5 thereof. It comprises a back plate B, apair of brake shoes L and T, a fixed abutment A and a double ended brakesleeve cylinder for applying the service braking load to the adjacenttips of the respective brake shoes.

The auxiliary (parking) brake load is applied through a handbrakemechanism including a cross-pull lever 120 having a lug 120A engaging aslipper plate TS in a window with trailing shoe T. The lever 120 ismounted on a shaft 121 which extends generally parallel with the backplate and which has its lower end supported in a bracket 122 permittingrotation of the shaft and lever and also permitting a degree ofarticulation of the lever about an axis extending parallel with the mainaxis of the brake drum.

The shaft 121 also engages one end of a strut 6 comprising a screw 7, anut 8 formed with a ratchet wheel 8A and a tube 9, which at its outerend bears against the leading shoe L. As so far described, it will beunderstood that rotation of lever 120 in the anticlockwise direction (asviewed in FIG. 2) will cause lug A to push the trailing shoe outwardly(to the right), the reaction causing shaft 121 to move in the oppositedirection, this movement being transferred to the leading shoe L by thestrut 6 extending between the shaft and the leading shoe.

A pawl 10 is also mounted in the shaft 121 for rotation about the axisthereof and has a lug 10B engaging the slipper plate TS. A coil torsionspring 12, surrounding the shaft 121 has one end anchored to the shaftby a pin 121A and at its other end acts on the pawl 10. The spring ispre-stressed in torsion in a sense to apply a continuous biassing forceto the pawl 10 tending to rotate the pawl in the anti-clockwisedirection (as viewed in FIG. 2), the direction in which the pawl movesto rotate the ratchet wheel 8A and thereby increase the length of strut6 to compensate for wear of the friction lining. Such rotation of thepawl is normally resisted by the reaction of the brake return springs inthe brakes off condition.

The spring 12 is also pre-stressed in compression to urge the pawlaxially of the shaft 121, into engagement with the adjacent surface oflever 120. As best seen in FIG. 5, the pawl has an offset portion 10Cbearing against the lever, to provide a rocking fulcrum for the pawlabout an axis extending transversely to the rotational axis of the pawl.Thus, as viewed in FIG. 5, the pawl is urged to rock in theanti-clockwise direction, so that the tip of the pawl is pressedyieldingly into engagement with the ratchet wheel.

No adjustment is effected during handbrake operation since thefrictional resistance to relative rotation of the strut parts is toogreat for the torsion in the coil spring 10, which therefore yields topermit the pawl lug 10A to separate from the slipper plate TS. This highfrictional restraint arises because the strut is loaded by the shoereturn springs throughout handbrake operation.

In service brake operation, however, the strut is relieved of the returnspring load, and outward movement of the shoe T permits the pawl 10 torotate under the action of the spring 12 to effect any increase in thelength of the strut by rotation of the ratchet wheel 8A. This conditionis illustrated in FIG. 4. Upon brake release, the pawl is able to rideover the crests of the ratchet teeth by rocking about its fulcrum 10C,against the restoring action of the spring 12.

In addition to its two primary functions described above, the spring 12serves the further useful function of maintaining the adjustment strutin contact with the further brake shoe L. The pawl acts, through its lug10C on the brake shoe T, the reaction being transmitted through theshaft 121 to the strut 6, which finds its own position of equilibrium byrocking in its mounting bracket 122.

The use of a coil torsion spring to apply the operating biassing forceto the pawl has the further advantage that such a spring is readilydesigned to have a low spring rate and to have a precisely set pre-loadin torsion. It can also be very compact and readily mounted, which isadvantageous when space is at a premium.

I claim:

1. An internal shoe drum brake including a pair of brake shoes which canbe expanded, against the action of shoe return spring means,alternatively by power actuated service brake means or by manuallyactuated auxiliary brake means, a strut of adjustable length whichtransmits manually applied loads between the shoes but is relieved ofload upon actuation of the service brake means, the strut comprising apair of relatively rotatably members screw-threaded together, and pawland ratchet means for effecting relative rotation of the members tolengthen the strut in response to excess travel of the brake shoes uponbrake actuation, wherein such relative rotation is effected only whenthe service brake is operated to relieve the load imposed on the strutby the return spring means, wherein a pawl spring in the form of a coilspring pre-stressed in torsion and in compression acts on the pawl onthe one hand to bias it to rotate in a sense to increase the length ofthe strut and on the other hand to urge the pawl laterally intoengagement with the ratchet, the rotational force applied to the pawl bythe spring being intermediate the force required to overcome thefrictional resistance to rotation of the strut parts when the strut issubjected to load by the shoe return springs and the force required toeffect such rotation when the shoe return spring load is relieved byservice brake actuation.

2. A brake according to claim 1, wherein the pawl and a brake applyinglever forming part of the manually actuated auxiliary brake means aremounted upon a shaft extending generally parallel to the back plate ofthe brake, for rotation about the axis of the shaft, the

' said shaft being mounted at its opposite end for articu-

1. An internal shoe drum brake including a pair of brake shoes which canbe expanded, against the action of shoe return spring means,alternatively by power actuated service brake means or by manuallyactuated auxiliary brake means, a strut of adjustable length whichtransmits manually applied loads between the shoes but is relieved ofload upon actuation of the service brake means, the strut comprising apair of relatively rotatably members screw-threaded togEther, and pawland ratchet means for effecting relative rotation of the members tolengthen the strut in response to excess travel of the brake shoes uponbrake actuation, wherein such relative rotation is effected only whenthe service brake is operated to relieve the load imposed on the strutby the return spring means, wherein a pawl spring in the form of a coilspring pre-stressed in torsion and in compression acts on the pawl onthe one hand to bias it to rotate in a sense to increase the length ofthe strut and on the other hand to urge the pawl laterally intoengagement with the ratchet, the rotational force applied to the pawl bythe spring being intermediate the force required to overcome thefrictional resistance to rotation of the strut parts when the strut issubjected to load by the shoe return springs and the force required toeffect such rotation when the shoe return spring load is relieved byservice brake actuation.
 2. A brake according to claim 1, wherein thepawl and a brake applying lever forming part of the manually actuatedauxiliary brake means are mounted upon a shaft extending generallyparallel to the back plate of the brake, for rotation about the axis ofthe shaft, the said shaft being mounted at its opposite end forarticulating movement about an axis extending parallel with the axis ofrotation of the brake drum and serving to transmit handbrake actuatingforces from the brake applying lever to the said strut.